Example Output

Synchronous time for 8 connections: 00:00:01.0194025
Synchronous time for 9 connections: 00:00:02.0362007
Asynchronous time for 8 connections: 00:00:01.0413737
Asynchronous time for 9 connections: 00:00:01.0238674

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace MvcApplication2
{
    // Note: For instructions on enabling IIS6 or IIS7 classic mode, 
    // visit http://go.microsoft.com/?LinkId=9394801

    public class WebApiApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            // Begin Changes
            int workerThreads, ioThreads;
            ThreadPool.GetMaxThreads(out workerThreads, out ioThreads);
            ThreadPool.SetMaxThreads(Environment.ProcessorCount, ioThreads);
            // End Changes

            AreaRegistration.RegisterAllAreas();

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
        }
    }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            MainAsync().Wait();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }

        Console.ReadKey();
    }

    static async Task MainAsync()
    {
        ServicePointManager.DefaultConnectionLimit = int.MaxValue;

        int connections;
        var syncUrl = "http://localhost:35697/api/values/";
        var asyncUrl = "http://localhost:35697/api/values/13";

        connections = Environment.ProcessorCount;
        Console.WriteLine(" Synchronous time for " + connections + " connections: " +
            await RunTest(syncUrl, connections));

        connections = Environment.ProcessorCount + 1;
        Console.WriteLine(" Synchronous time for " + connections + " connections: " +
            await RunTest(syncUrl, connections));

        connections = Environment.ProcessorCount;
        Console.WriteLine("Asynchronous time for " + connections + " connections: " +
            await RunTest(asyncUrl, connections));

        connections = Environment.ProcessorCount + 1;
        Console.WriteLine("Asynchronous time for " + connections + " connections: " +
            await RunTest(asyncUrl, connections));
    }

    static async Task<TimeSpan> RunTest(string url, int concurrentConnections)
    {
        var sw = new Stopwatch();
        var client = new HttpClient();

        await client.GetStringAsync(url); // warmup
        sw.Start();
        await Task.WhenAll(Enumerable.Range(0, concurrentConnections).Select(i => client.GetStringAsync(url)));
        sw.Stop();

        return sw.Elapsed;
    }
}

ValuesController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Web.Http;

namespace MvcApplication2.Controllers
{
    public class ValuesController : ApiController
    {
        // Synchronous
        public IEnumerable<string> Get()
        {
            Thread.Sleep(1000);
            return new string[] { "value1", "value2" };
        }

        // Asynchronous
        public async Task<IEnumerable<string>> Get(int id)
        {
            await Task.Delay(1000);
            return new string[] { "value1", "value2" };
        }
    }
}